Modern automotive engines typically utilize a catalytic converter to reduce regulated exhaust gases produced by the engine. Such converters operate to chemically alter the exhaust gas composition produced by the engine. Catalytic converters typically operate at peak efficiency when the temperature of the catalytic material within the converter is within a certain specified range. Continued operation of the converter at a temperature greater than the specified temperature range, however, leads to degradation of the catalytic converter material within the converter. Such degradation leads to reduced converter operating life.
One method to limit the converter temperature is to ramp the engine air/fuel ratio injected into the engine to a value rich of stoichiometry that will maintain the temperature at the maximum when the conditions are such that normal stoichiometric operation would cause the temperature to exceed the maximum allowable temperature. The ramping rate of the air/fuel ratio that is injected is a fixed value and is large enough that the converter temperature does not exceed the maximum allowable temperature. Further, a single predetermined temperature limit is used. Such a system is described in U.S. Pat. No. 5,414,994.
The inventors herein have recognized a disadvantage with the above approach. During certain operating conditions, especially high load and high speed operation, the necessary enrichment to maintain the converter temperature at or below the predetermined temperature limit causes an increase in certain regulated exhaust gases. Further, the enrichment also decreases fuel economy.